This project involves a comparative approach to the ultrastructure of spermatozoa, principally of invertebrates, with emphasis on gametes of turbellarian flatworms and of molluscs. Such features as (1) patterns of microtubules in axonemes, (2) chromatin condensation during spermiogenesis, (3) cytoplasmic inclusions in sperm, including refractile bodies, "enigmatic Bodies," acrosomal constituents, mitochondria, lysosomes, etc., (4) particulate structures in and on sperm membrances which can be revealed by freeze-fracture techniques, (5) mode of locomotion in relation to number of axonemes per spermatozoon and type of central structure in the axoneme, and (6) the eupyrene, oligopyrene or apyrene nature of the spermatozoon where these differences exist, are being examined. In addition to phase contrast studies on living material, use is being made of thin-sectioning, negative staining and freeze-etch techniques of electron microscopy. A growing body of evidence has suggested a new theory to account for the basic mechanics of ciliary and flagellar motility, and to indicate a morphological basis for the differences between planar and helical movements of flagella. The physical appearance of the supercoiled chromatin strands as delineated in freeze-etch preparations of condensing spermatid nuclei of the snail Goniobasis has now been revealed. In addition, this same matebial (Goniobasis) is being utilized to study the atypical (oligopyrene and apyrene) spermatozoa and the remarkable changes in cytoplasmic constituents and in the chromosomes and chromosomal spindle fibers taking place within them.